1. Technical Field
Exemplary aspects of the present invention relate to a fixing device for fixing an image with heat and pressure on a recording material conveyed from an upstream side of a nipping portion formed between two rotary bodies pressing against each other, and more particularly, to a recording medium separation configuration of the fixing device. Exemplary aspects of the present invention also relate to an image forming apparatus including the fixing device.
2. Description of the Related Art
Image forming apparatuses are used as, for example, copiers, printers, and facsimile machines employing electrophotographic techniques, or multifunction peripherals having at least one of copying, printing, and facsimile functions. Such electrophotographic image forming apparatuses may employ a heat fixing device. Such a heat fixing device fixes a toner image on a recording medium by fusing and pressing the toner image transferred to the recording medium. The heat fixing device allows the recording medium to pass a nipping portion formed between a heating rotary body heated by a heat source and a pressure rotary body pressing against the heating rotary body, so that the toner image is fixed on the recording medium.
When toner is fused, the toner becomes adhesive. The adhesiveness of the toner may cause a recording medium bearing a toner image to be wound around the heating rotary body. Thus, an outer surface of the heating rotary body includes a release layer made of a material, such as fluorine resin and silicone rubber, as a surface layer. The release layer prevents the recording medium bearing the toner image from winding around the heating rotary body due to the adhesiveness of toner.
Moreover, a separation unit including a separation pawl is attached adjacent to the heating rotary body to forcibly separate the recording medium, which tends to be wound around the heating rotary body due to the fused toner, from the heating rotary body. FIG. 8 is a partial view of one example of such a separation unit. A separation member 5 serving as a separation unit is arranged on a downstream side of a heat fixing device 1 in a recording medium conveyance direction. The heat fixing device 1 includes a heating roller 2 heated by a heater and a pressure roller 3 disposed opposite to the heating roller 2. The separation member 5 is made of heat-resistant resin and rotatably supported around a fulcrum 5b. The separation member 5 includes a separation pawl 5a extending near an exit of a nipping portion formed between the heating roller 2 and the pressure roller 3 pressing against each other. A spring 4 applies pressure to the separation pawl 5a such that the separation pawl 5a contacts the heating roller 2. The separation pawl 5a is coated with fluorine resin to prevent abrasion caused by contact with the heating roller 2. The separation pawl 5a has a pawl width of approximately 2 mm to approximately 10 mm, and a plurality of separation pawls 5a are arranged within a sheet-passing area.
Since the separation pawl 5a is in contact with the heating roller 2, a portion of the separation pawl 5a contacting the heating roller 2 is abraded. Such abrasion may cause abrasion damage to a toner image. Moreover, the separation pawl 5a has a tip having an angle limit of approximately 20 degrees in manufacturing, causing difficulty in arranging the tip of the separation pawl 5a nearer to the exit of the nipping portion. A leading edge of a recording medium is likely to be wound around the heating roller 2 while being conveyed from the exit of the nipping portion to the tip of the separation pawl 5a. Thus, the longer the distance between the exit of the nipping portion and the tip of the separation pawl 5a, the more likely an excess amount of heat is to be applied to an image on the leading edge of the recording medium. Consequently, the excess amount of heat may cause generation of an irregular image.
Hence, as illustrated in FIG. 9, a separation plate 6 serving as a separation member may be employed. The separation plate 6 includes a metal plate frame 6a having a rotation fulcrum 6d, a contact member 6b made of heat-resistant resin, and a thin plate 6c having a thickness of approximately 0.3 mm. The thin plate 6c connects the metal plate frame 6a and the contact member 6b. A spring 4 applies pressure such that the thin plate 6c contacts a heating roller 2. The thin plate 6c extends across an entire sheet-passing area, and the contact member 6b adjusts a clearance between the heating roller 2 and the thin plate 6c such that the clearance becomes approximately 0.5 mm. The separation plate 6 includes projected members arranged on both ends thereof in a longitudinal direction (a direction perpendicular to a sheet surface), and a position of the separation plate 6 is adjusted and fixed by a jig, for example. The clearance between the heating roller 2 and the thin plate 6c is changed by adjusting the position of the separation plate 6, whereas the clearance is determined by fixing the position of the separation plate 6. The thin plate 6c has a leading edge that is arranged near an exit of the nipping portion. Thus, the separation plate 6 reliably separates a recording medium from the heating roller 2 before the recording medium is wound around the heating roller 2 if the recording medium has a few-millimeter blank (an outer margin) in a leading edge thereof. Since the thin plate 6c rubs against the recording medium, a surface of the thin plate 6c is coated with fluorine resin, thereby preventing adhesion of toner to the thin plate 6c. 
The thin plate 6c of the separation plate 6 is arranged near the exit of the nipping portion, but not in contact with the heating roller 2. Such an arrangement of the thin plate 6c can prevent the problem of the separation pawl in the configuration described with reference to FIG. 8. Moreover, a configuration of enabling adjustment of the clearance is proposed for such a separation plate 6. The proposed configuration enables the clearance to be adjusted during assembly, so that the clearance can be narrower.
Alternatively, JP-2010-079219-A proposes an arrangement of a leading edge of a separation member in a non-contact state to a position where the separation member can maintain separability which is substantially the same as or better than that of the above-described separation pawl. Such an arrangement is made in response to fluctuations of a rotary-body surface in the immediate vicinity on a downstream side of a nipping portion such that the leading edge of the separation member can be disposed in the immediate vicinity on the downstream side of the nipping portion and adjacent to the rotary-body surface. According to this configuration, a separation plate holder 8 as partially illustrated in FIG. 10 is arranged with a separation plate and a guide member. The separation plate holder 8 is a metal plate. Such a separation plate holder 8 holds the separation member such that the height of the separation member is adjustable in a screw tightening direction perpendicular to a recording medium conveyance direction by a spring and a screw.
In the separation plate holder 8, both ends of a base body (both ends of the holder in a longitudinal direction) are bent at a right angle to provide flange surfaces 8′, and rotation fulcrum pins 9 are swaged to the respective flange surfaces 8′. The separation plate holder 8 has a side along the longitudinal direction thereof. The side is also bent at a right angle, and both ends thereof are further bent at a right angle to serve as positioning surfaces 8″ to which positioning members can be fastened.
The separation plate holder 8 rotates around the rotation fulcrum pins 9, and the positioning members contact a fixing member that is a fixing roller or a fixing belt, so that the separation plate holder 8 is positioned with the fixing member. Since the separation plate is adjustably held with respect to the separation plate holder 8, a clearance with respect to the fixing member can be adjusted by the separation plate, thereby obtaining a narrower clearance. If the clearance is narrower, there is an advantage that the separation plate can more readily separate a recording medium from the fixing member. The separation plate can separate even a thin recording medium from the fixing member. If the clearance is wider, on the other hand, the recording medium can slip into the clearance more easily, causing an increase in the likelihood of recording medium jams. Thus, the clearance is preferably set as narrow as possible.
In such a separation unit, positional accuracy of a rotation fulcrum and a positioning surface of the separation plate holder is important. The rotation fulcrums and the positioning surfaces are arranged on both sides of the separation plate holder in a longitudinal direction (a rotation axis direction). However, in a case where the positional accuracy of these members is not good, only one of the positioning surfaces contacts the fixing member, causing the separation plate holder to be poorly positioned with respect to the fixing member.
In the holder configuration as illustrated in FIG. 10, however, since three bending processes need to be performed when an area from the flange surface for the rotation fulcrum to the positioning surface is formed, the positional accuracy of each of the members is difficult to obtain. Moreover, in a case where such a separation plate holder is used in an actual device, the separation plate might be easily deformed by an external force due to a small dimension of a bent portion thereof that has a dimension of approximately 10 mm.
If a recording medium is jammed and stuck in an accordion shape in the vicinity of the separation plate (the separation member), there are cases where a load is generated to the separation plate holder through the separation plate, and a bending portion of the separation plate holder is deformed. In such cases, the deformed separation plate holder cannot be positioned in a target position with respect to the fixing member, causing a change in the clearance of the separation plate. Consequently, the separation plate may damage the fixing member by interfering with the fixing member, or cannot separate the recording medium from the fixing member due to a wider clearance.